According to Associated Press, One Direction's Publicist Simon Jones announced that Zayn "has signed off with stress and is flying back to the U.K to recuperate". The group still has the Southeast Asia tour up ahead with a scheduled event at South Africa at the end of this month.
But there's also another side to the story - rumors have circulated that there is trouble in paradise between Zayn and fiancé, Perry Edwards as images of Zayn and another woman surfaced the web.
So there's a lot of reasons to answer in the question, but the real answer, Zayn only knows. For now, we hope he can rest away, de stress in bliss.